Gαo1 and Gαo1/Gαo2 deletion differentially affect hippocampal mossy fiber tract anatomy and neuronal morphogenesis.

The heterotrimeric G-protein αo subunit is ubiquitously expressed in the CNS as two splice variants Gαo1 and Gαo2, regulating various brain functions. Here, we investigated the effect of single Gαo1, Gαo2, and double Gαo1/2 knockout on the postnatal development of the murine mossy fiber tract, a central pathway of the hippocampal connectivity circuit. The size of the hippocampal synaptic termination fields covered by mossy fiber boutons together with various fiber length parameters of the tract was analyzed by immunohistochemical staining of the vesicular Zinc transporter 3 (ZnT3) or Synaptoporin at postnatal days 2, 4, 8, 12, 16, and in the adult. Ultimately, Gαo1 knockout resulted in a reduced developmental growth of synaptic mossy fiber terminal fields by 37% in the adult Stratum lucidum and by 30% in the total mossy fiber tract size. Other morphological parameters such as projection length of the infrapyramidal bundle of the tract were increased (+52% in Gαo1 -/- mice). In contrast, Gαo2 knockout had no effects on the mossy fiber tract. Moreover, by using primary heterozygous and homozygous Gαo1 knockout hippocampal cultures, we detected a strongly pronounced reduction in axon and dendrite length (-50% and -38%, respectively) as well as axon and dendrite arborization complexity (-75% and -72% branch nodes, respectively) in the homozygous knockout. Deletion of both splice variants Gαo1 and Gαo2 partially rescued the in vivo and completely reconstituted the in vitro effects, indicating an opposing functional relevance of the two Gαo splice variants for neuronal development and synaptic connectivity.

Sex and brain region-specific regulation of serotonin transporter activity in synaptosomes in guanine nucleotide-binding protein G(q) alpha knockout mice.

The regulation of the serotonin transporter (SERT) by guanine nucleotide-binding protein alpha (Gα) q was investigated using Gαq knockout mice. In the absence of Gαq, SERT-mediated uptake of 5-hydroxytryptamine (5HT) was enhanced in midbrain and frontal cortex synaptosomes, but only in female mice. The mechanisms underlying this sexual dimorphism were investigated using quantitative western blot analysis revealing brain region-specific differences. In the frontal cortex, SERT protein expression was decreased in male knockout mice, seemingly explaining the sex-dependent variation in SERT activity. The differential expression of Gαi1 in female mice contributes to the sex differences in the midbrain. In fact, Gαi1 levels inversely correlate with 5HT uptake rates across both sexes and genotypes. Likely due to differential SERT regulation as well as sex differences in the expression of tryptophan hydroxylase 2, Gαq knockout mice also displayed sex- and genotype-dependent alterations in total 5HT tissue levels as determined by high-performance liquid chromatography. Gαq inhibitors, YM-254890 and BIM-46187, differentially affected SERT activity in both, synaptosomes and cultured cells. YM-254890 treatment mimicked the effect of Gαq knockout in the frontal cortex. BIM-46187, which promotes the nucleotide-free form of Gα proteins, substantially inhibited 5HT uptake, prompting us to hypothesise that Gαq interacts with SERT similarly as with G-protein-coupled receptors and inhibits SERT activity by modulating transport-associated conformational changes. Taken together, our findings reveal a novel mechanism of SERT regulation and impact our understanding of sex differences in diseases associated with dysregulation of serotonin transmission, such as depression and anxiety.

An Alternative Exon of CAPS2 Influences Catecholamine Loading into LDCVs of Chromaffin Cells.

The calcium-dependent activator proteins for secretion (CAPS) are priming factors for synaptic and large dense-core vesicles (LDCVs), promoting their entry into and stabilizing the release-ready state. A modulatory role of CAPS in catecholamine loading of vesicles has been suggested. Although an influence of CAPS on monoamine transporter function and on vesicle acidification has been reported, a role of CAPS in vesicle loading is disputed. Using expression of naturally occurring splice variants of CAPS2 into chromaffin cells from CAPS1/CAPS2 double-deficient mice of both sexes, we show that an alternative exon of 40 aa is responsible for enhanced catecholamine loading of LDCVs in mouse chromaffin cells. The presence of this exon leads to increased activity of both vesicular monoamine transporters. Deletion of CAPS does not alter acidification of vesicles. Our results establish a splice-variant-dependent modulatory effect of CAPS on catecholamine content in LDCVs.SIGNIFICANCE STATEMENT The calcium activator protein for secretion (CAPS) promotes and stabilizes the entry of catecholamine-containing vesicles of the adrenal gland into a release-ready state. Expression of an alternatively spliced exon in CAPS leads to enhanced catecholamine content in chromaffin granules. This exon codes for 40 aa with a high proline content, consistent with an unstructured loop present in the portion of the molecule generally thought to be involved in vesicle priming. CAPS variants containing this exon promote serotonin uptake into Chinese hamster ovary cells expressing either vesicular monoamine transporter. Epigenetic tuning of CAPS variants may allow modulation of endocrine adrenaline and noradrenaline release. This mechanism may extend to monoamine release in central neurons or in the enteric nervous system.

Release of astroglial vimentin by extracellular vesicles: Modulation of binding and internalization of C3 transferase in astrocytes and neurons.

Clostridium botulinum C3 transferase (C3bot) ADP-ribosylates rho proteins to change cellular functions in a variety of cell types including astrocytes and neurons. The intermediate filament protein vimentin as well as transmembrane integrins are involved in internalization of C3bot into cells. The exact contribution, however, of these proteins to binding of C3bot to the cell surface and subsequent cellular uptake remains to be unraveled. By comparing primary astrocyte cultures derived from wild-type with Vim-/- mice, we demonstrate that astrocytes lacking vimentin exhibited a delayed ADP-ribosylation of rhoA concurrent with a blunted morphological response. This functional impairment was rescued by the extracellular excess of recombinant vimentin. Binding assays using C3bot harboring a mutated integrin-binding RGD motif (C3bot-G89I) revealed the involvement of integrins in astrocyte binding of C3bot. Axonotrophic effects of C3bot are vimentin dependent and postulate an underlying mechanism entertaining a molecular cross-talk between astrocytes and neurons. We present functional evidence for astrocytic release of vimentin by exosomes using an in vitro scratch wound model. Exosomal vimentin+ particles released from wild-type astrocytes promote the interaction of C3bot with neuronal membranes. This effect vanished when culturing Vim-/- astrocytes. Specificity of these findings was confirmed by recombinant vimentin propagating enhanced binding of C3bot to synaptosomes from rat spinal cord and mouse brain. We hypothesize that vimentin+ exosomes released by reactive astrocytes provide a novel molecular mechanism constituting axonotrophic (neuroprotective) and plasticity augmenting effects of C3bot after spinal cord injury.

The Rho ADP-ribosylating C3 exoenzyme binds cells via an Arg-Gly-Asp motif.

The Rho ADP-ribosylating C3 exoenzyme (C3bot) is a bacterial protein toxin devoid of a cell-binding or -translocation domain. Nevertheless, C3 can efficiently enter intact cells, including neurons, but the mechanism of C3 binding and uptake is not yet understood. Previously, we identified the intermediate filament vimentin as an extracellular membranous interaction partner of C3. However, uptake of C3 into cells still occurs (although reduced) in the absence of vimentin, indicating involvement of an additional host cell receptor. C3 harbors an Arg-Gly-Asp (RGD) motif, which is the major integrin-binding site, present in a variety of integrin ligands. To check whether the RGD motif of C3 is involved in binding to cells, we performed a competition assay with C3 and RGD peptide or with a monoclonal antibody binding to ß1-integrin subunit and binding assays in different cell lines, primary neurons, and synaptosomes with C3-RGD mutants. Here, we report that preincubation of cells with the GRGDNP peptide strongly reduced C3 binding to cells. Moreover, mutation of the RGD motif reduced C3 binding to intact cells and also to recombinant vimentin. Anti-integrin antibodies also lowered the C3 binding to cells. Our results indicate that the RGD motif of C3 is at least one essential C3 motif for binding to host cells and that integrin is an additional receptor for C3 besides vimentin.

Balance of Go1α and Go2α expression regulates motor function via the striatal dopaminergic system.

The heterotrimeric G-protein Go with its splice variants, Go1α and Go2α, seems to be involved in the regulation of motor function but isoform-specific effects are still unclear. We found that Go1α-/- knockouts performed worse on the rota-rod than Go2α-/- and wild-type (WT) mice. In Go1+2α-/- mice motor function was partially recovered. Furthermore, Go1+2α-/- mice showed an increased spontaneous motor activity. Compared to wild types or Go2α-/- mice, Go1+2α-/- mice developed increased behavioural sensitization following repetitive cocaine treatment, but failed to develop conditioned place preference. Analysis of dopamine concentration and expression of D1 and D2 receptors unravelled splice-variant-specific imbalances in the striatal dopaminergic system: In Go1α-/- mice dopamine concentration and vesicular monoamine uptake were increased compared to wild types. The expression of the D2 receptor was higher in Go1α-/- compared to wild type littermates, but unchanged in Go2α-/- mice. Deletion of both Go1α and Go2α re-established both dopamine and D2 receptor levels comparable to those in the wild-type. Cocaine treatment had no effect on the ratio of D1 receptor to D2 receptor in Go1+2α-/- mutants, but decreased this ratio in Go2α-/- mice. Finally, we observed that the deletion of Go1α led to a threefold higher striatal expression of Go2α. Taken together our data suggest that a balance in the expression of Go1α and Go2α sustains normal motor function. Deletion of either splice variant results in divergent behavioural and molecular alterations in the striatal dopaminergic system. Deletion of both splice variants partially restores the behavioural and molecular changes. Open Data: Materials are available on https://cos.io/our-services/open-science-badges/ https://osf.io/93n6m/.

Synapsin-antibodies in psychiatric and neurological disorders: Prevalence and clinical findings.

OBJECTIVE: To study the prevalence of autoantibodies to synapsin in patients with psychiatric and neurological disorders and to describe clinical findings in synapsin antibody positive patients. METHODS: Sera of 375 patients with different psychiatric and neurological disorders and sera of 97 healthy controls were screened (dilution 1:320) for anti-synapsin IgG using HEK293 cells transfected with rat synapsin Ia. Positive sera were further analyzed by immunoblots with brain tissue from wild type and synapsin knock out mice and with HEK293 cells transfected with human synapsin Ia and Ib. Binding of synapsin IgG positive sera to primary neurons was studied using murine hippocampal neurons. RESULTS: IgG in serum from 23 (6.1%) of 375 patients, but from none of the 97 healthy controls (p=0.007), bound to rat synapsin Ia transfected cells with a median (range) titer of 1:1000 (1:320-1:100,000). Twelve of the 23 positive sera reacted with a protein of the molecular size of synapsin I in immunoblots of wild type but not of synapsin knock out mouse brain tissue. Out of 19/23 positive sera available for testing, 13 bound to human synapsin Ia and 16 to human synapsin Ib transfected cells. Synapsin IgG positive sera stained fixed and permeabilized murine hippocampal neurons. Synapsin IgG positive patients had various psychiatric and neurological disorders. Tumors were documented in 2 patients (melanoma, small cell lung carcinoma); concomitant anti-neuronal or other autoantibodies were present in 8 patients. CONCLUSIONS: Autoantibodies to human synapsin Ia and Ib are detectable in a proportion of sera from patients with different psychiatric and neurological disorders, warranting further investigation into the potential pathophysiological relevance of these antibodies.

The intermediate filament protein vimentin is essential for axonotrophic effects of Clostridium botulinum C3 exoenzyme.

The type III intermediate filament protein vimentin was recently identified to mediate binding and uptake of Clostridium botulinum C3 exoenzyme (C3bot) in two cell lines. Here, we used primary neuronal cultures from vimentin knockout (Vim-/- ) mice to study the impact of vimentin on axonal growth and internalization of C3bot. In contrast to wild type, vimentin knockout neurons were insensitive to C3bot. Application of extracellular vimentin to Vim-/- neurons completely restored the growth-promoting effects of C3bot. In line with this uptake of C3bot into Vim-/- neurons was strongly decreased resulting in reduced ADP-ribosylation of RhoA and B as detected by an antibody recognizing selectively ADP-ribosylated RhoA/B. Again, uptake of C3bot into Vim-/- neurons was rescued by addition of extracellular vimentin. In addition, in purified embryonic stem cell-derived motor neurons that are devoid of glial cells C3bot elicited axonotrophic effects confining neuronal vimentin as a binding partner. Primary neuronal cultures from vimentin knockout (KO) mice were used to study the impact of vimentin on axonal growth and internalization of C3bot. In contrast to wild type, vimentin knockout neurons were insensitive to the axonotrophic effects of C3bot. Application of extracellular vimentin (recombinant vimentin) to vimentin KO neurons completely restored the growth-promoting effects of C3bot. In line with this uptake of C3bot into vimentin KO neurons was strongly decreased resulting in reduced ADP-ribosylation of RhoA and B as detected by an antibody recognizing selectively ADP-ribosylated RhoA/B.

Titration of Syntaxin1 in mammalian synapses reveals multiple roles in vesicle docking, priming, and release probability.

Synaptic vesicles undergo sequential steps in preparation for neurotransmitter release. Individual SNARE proteins and the SNARE complex itself have been implicated in these processes. However, discrete effects of SNARE proteins on synaptic function have been difficult to assess using complete loss-of-function approaches. We therefore used a genetic titration technique in cultured mouse hippocampal neurons to evaluate the contribution of the neuronal SNARE protein Syntaxin1 (Stx1) in vesicle docking, priming, and release probability. We generated graded reductions of total Stx1 levels by combining two approaches, namely, endogenous hypomorphic expression of the isoform Stx1B and RNAi-mediated knockdown. Proximity of synaptic vesicles to the active zone was not strongly affected. However, overall release efficiency of affected neurons was severely impaired, as demonstrated by a smaller readily releasable pool size, slower refilling rate of primed vesicles, and lower release probability. Interestingly, dose-response fitting of Stx1 levels against readily releasable pool size and vesicular release probability showed similar Kd (dissociation constant) values at 18% and 19% of wild-type Stx1, with cooperativity estimates of 3.4 and 2.5, respectively. This strongly suggests that priming and vesicle fusion share the same molecular stoichiometry, and are governed by highly related mechanisms.

Synaptic vesicle proteins: targets and routes for botulinum neurotoxins.

Synaptic vesicles (SV) are key organelles of neuronal communication. SV are responsible for the storage of neurotransmitters, which are released by Ca(2+)-dependent exocytosis. After release and interaction with postsynaptic receptors, transmitters rapidly diffuse out of the synaptic cleft and are sequestered by plasma membrane transporters (in some cases following enzymatic conversion). SVs undergo endocytosis and are refilled by specific vesicular transmitter transporters different in the various neuronal subtypes. Besides these differences, SVs in general are equipped with a remarkable common set of proteins. Botulinum neurotoxins (BoNTs) inhibit neurotransmitter release from almost all types of neurons by cleaving proteins required for membrane fusion localized either to SVs (synaptobrevin) or to the plasma membrane (SNAP-25 and syntaxin) depending on the BoNT serotype. To enter the neuronal cytoplasm, BoNTs specifically interact with the luminal domain of SV proteins (synaptotagmin or SV2, depending on serotype) transiently exposed during exocytotic membrane fusion and occurring in almost every neuron. Thus, the highly specific interaction with luminal domains of SV proteins commonly expressed on all SV types is one reason why BoNTs exhibit such a high neuronal specificity but attack almost every neuron type.

The α-subunit of the trimeric GTPase Go2 regulates axonal growth.

The Goα splice variants Go1α and Go2α are subunits of the most abundant G-proteins in brain, Go1 and Go2. Only a few interacting partners binding to Go1α have been described so far and splice variant-specific differences are not known. Using a yeast two-hybrid screen with constitutively active Go2α as bait, we identified Rap1GTPase activating protein (Rap1GAP) and Girdin as interacting partners of Go2α, which was confirmed by co-immunoprecipitation. Comparison of subcellular fractions from brains of wild type and Go2α-/- mice revealed no differences in the overall expression level of Girdin or Rap1GAP. However, we found higher amounts of active Rap1-GTP in brains of Go2α deficient mutants, indicating that Go2α may increase Rap1GAP activity, thereby effecting the Rap1 activation/deactivation cycle. Rap1 has been shown to be involved in neurite outgrowth and given a Rap1GAP-Go2α interaction, we found that the loss of Go2α affected axonal outgrowth. Axons of cultured cortical and hippocampal neurons prepared from embryonic Go2α-/- mice grew longer and developed more branches than those from wild-type mice. Taken together, we provide evidence that Go2α regulates axonal outgrowth and branching.

Activation of metabotropic GABA receptors increases the energy barrier for vesicle fusion.

Neurotransmitter release from presynaptic terminals is under the tight control of various metabotropic receptors. We report here that in addition to the regulation of Ca(2+) channel activity, metabotropic GABA(B) receptors (GABA(B)Rs) at murine hippocampal glutamatergic synapses utilize an inhibitory pathway that directly targets the synaptic vesicle release machinery. Acute application of the GABA(B)R agonist baclofen rapidly and reversibly inhibits vesicle fusion, which occurs independently of the SNAP-25 C-terminus. Using applications of hypertonic sucrose solutions, we find that the size of the readily releasable pool remains unchanged by GABA(B)R activation, but the sensitivity of primed vesicles to hypertonic stimuli appears lowered as the response amplitudes at intermediate sucrose concentrations are smaller and release kinetics are slowed. These data show that presynaptic GABA(B)Rs can inhibit neurotransmitter release directly by increasing the energy barrier for vesicle fusion.

N-methyl-D-aspartate receptor antibodies in herpes simplex encephalitis.

OBJECTIVE: To determine the presence and kinetics of antibodies against synaptic proteins in patients with herpes simplex virus encephalitis (HSE). METHODS: Retrospective analysis of 44 patients with polymerase chain reaction-proven HSE for the presence of a large panel of onconeuronal and synaptic receptor antibodies. The effect of patients' serum was studied in cultures of primary mouse hippocampal neurons. RESULTS: N-Methyl-D-aspartate receptor (NMDAR) antibodies of the immunoglobulin (Ig) subtypes IgA, IgG, or IgM were detected in 13 of 44 patients (30%) in the course of HSE, suggesting secondary autoimmune mechanisms. NMDAR antibodies were often present at hospital admission, but in some patients developed after the first week of HSE. Antibody-positive sera resulted in downregulation of synaptic marker proteins in hippocampal neurons. INTERPRETATION: Some patients with HSE develop IgA, IgG, or IgM autoantibodies against NMDAR. Sera from these patients alter the density of neuronal synaptic markers, suggesting a potential pathogenic disease-modifying effect. These findings have implications for the understanding of autoimmunity in infectious diseases, and prospective studies should reveal whether the subgroup of patients with HSE and NMDAR antibodies may benefit from immunotherapy. .

Synaptophysin is involved in resetting of the mammalian circadian clock.

BACKGROUND: Mammals can adapt to changing light/dark conditions by advancing or delaying their circadian clock phase. Light pulses evoke changes in gene expression and neuronal activity in the suprachiasmatic nuclei (SCN), the central pacemaker of the circadian system. Alterations in neuronal activity are partially mediated by changes in synaptic vesicle (SV) fusion at the presynaptic membrane, which modulates release of neurotransmitters. METHODS: Male synaptophysin (Syp) knock-out and littermate control wild type mice were tested in an Aschoff type I resetting paradigm. Additionally, gene expression of cFos, Per1 and Per2 was assessed in the SCN. Finally, complexes between the synaptic vesicle proteins Syp and synaptobrevin (Syb) were studied in order to correlate behavior with protein complexes at synaptic vesicles. RESULTS: Here we show that mice lacking Syp, a modulator of neurotransmitter release, are defective in delaying clock phase. In contrast, clock phase advances as well as clock period are normal in Syp-/- knock-out mice. This correlates with the formation of Syp/Syb complexes. CONCLUSIONS: Our findings suggest that Syp is involved specifically in the response to a nocturnal light pulse occurring in the early night. It appears that the SV component Syp is critically involved in the delay portion of the resetting mechanism of the circadian clock.

Into great silence without VGLUT3.

The vesicular glutamate transporters VGLUT1 and VGLUT2 fill synaptic vesicles with glutamate, an essential prerequisite for glutamatergic transmission in the CNS. In contrast, the third isoform, VGLUT3, is not confined to glutamatergic neurons, and its function has remained enigmatic. In this issue of Neuron, Seal et al. show that mice lacking VGLUT3 are profoundly deaf and exhibit nonconvulsive seizures.

Minimal essential length of Clostridium botulinum C3 peptides to enhance neuronal regenerative growth and connectivity in a non-enzymatic mode.

C3 ADP-ribosyltransferase is a valuable tool to study Rho-dependent cellular processes. In the current study we investigated the impact of enzyme-deficient peptides derived from Clostridium botulinum C3 transferase in the context of neuronal process elongation and branching, synaptic connectivity, and putative beneficial effects on functional outcome following traumatic injury to the CNS. By screening a range of peptidic fragments, we identified three short peptides from C3bot that promoted axon and dendrite outgrowth in cultivated hippocampal neurons. Furthermore, one of these fragments, a 26-amino acid peptide covering the residues 156-181 enhanced synaptic connectivity in primary hippocampal culture. This peptide was also effective to foster axon outgrowth and re-innervation in organotypical brain slice culture. To evaluate the potential of the 26mer to foster repair mechanisms after CNS injury we applied this peptide to mice subjected to spinal cord injury by either compression impact or hemisection. A single local administration at the site of the lesion improved locomotor recovery. In addition, histological analysis revealed an increased serotonergic input to lumbar motoneurons in treated compared with control mice. Pull-down assays showed that lesion-induced up-regulation of RhoA activity within the spinal cord was largely blocked by C3bot peptides despite the lack of enzymatic activity.

C3 peptide enhances recovery from spinal cord injury by improved regenerative growth of descending fiber tracts.

Functional recovery and regeneration of corticospinal tract (CST) fibers following spinal cord injury by compression or dorsal hemisection in mice was monitored after application of the enzyme-deficient Clostridium botulinum C3-protein-derived 29-amino-acid fragment C3bot(154-182). This peptide significantly improved locomotor restoration in both injury models as assessed by the open-field Basso Mouse Scale for locomotion test and Rotarod treadmill experiments. These data were supported by tracing studies showing an enhanced regenerative growth of CST fibers in treated animals as visualized by anterograde tracing. Additionally, C3bot(154-182) stimulated regenerative growth of raphespinal fibers and improved serotonergic input to lumbar alpha-motoneurons. These in vivo data were confirmed by in vitro data, showing an enhanced axon outgrowth of alpha-motoneurons and hippocampal neurons cultivated on normal or growth-inhibitory substrates after application of C3bot(154-182). The observed effects were probably caused by a non-enzymatic downregulation of active RhoA by the C3 peptide as indicated by pull-down experiments. By contrast, C3bot(154-182) did not induce neurite outgrowth in primary cultures of dorsal root ganglion cells. In conclusion, C3bot(154-182) represents a novel, promising tool to foster axonal protection and/or repair, as well as functional recovery after traumatic CNS injury.

Enhancement of Phosphorylation and Transport Activity of the Neuronal Glutamate Transporter Excitatory Amino Acid Transporter 3 by C3bot and a 26mer C3bot Peptide.

In primary murine hippocampal neurons we investigated the regulation of EAAT3-mediated glutamate transport by the Clostridium botulinum C3 transferase C3bot and a 26mer peptide derived from full length protein. Incubation with either enzyme-competent C3bot or enzyme-deficient C3bot156-181 peptide resulted in the upregulation of glutamate uptake by up to 22% compared to untreated cells. A similar enhancement of glutamate transport was also achieved by the classical phorbol-ester-mediated activation of protein kinase C subtypes. Yet comparable, effects elicited by C3 preparations seemed not to rely on PKCα, γ, ε, or ζ activation. Blocking of tyrosine phosphorylation by tyrosine kinase inhibitors prevented the observed effect mediated by C3bot and C3bot 26mer. By using biochemical and molecular biological assays we could rule out that the observed C3bot and C3bot 26mer-mediated effects solely resulted from enhanced transporter expression or translocation to the neuronal surface but was rather mediated by transporter phosphorylation at tyrosine residues that was found to be significantly enhanced following incubation with either full length protein or the 26mer C3 peptide.

Quantitative comparison of glutamatergic and GABAergic synaptic vesicles unveils selectivity for few proteins including MAL2, a novel synaptic vesicle protein.

Synaptic vesicles (SVs) store neurotransmitters and release them by exocytosis. The vesicular neurotransmitter transporters discriminate which transmitter will be sequestered and stored by the vesicles. However, it is unclear whether the neurotransmitter phenotype of SVs is solely defined by the transporters or whether it is associated with additional proteins. Here we have compared the protein composition of SVs enriched in vesicular glutamate (VGLUT-1) and GABA transporters (VGAT), respectively, using quantitative proteomics. Of >450 quantified proteins, approximately 50 were differentially distributed between the populations, with only few of them being specific for SVs. Of these, the most striking differences were observed for the zinc transporter ZnT3 and the vesicle proteins SV2B and SV31 that are associated preferentially with VGLUT-1 vesicles, and for SV2C that is associated mainly with VGAT vesicles. Several additional proteins displayed a preference for VGLUT-1 vesicles including, surprisingly, synaptophysin, synaptotagmins, and syntaxin 1a. Moreover, MAL2, a membrane protein of unknown function distantly related to synaptophysins and SCAMPs, cofractionated with VGLUT-1 vesicles. Both subcellular fractionation and immunolocalization at the light and electron microscopic level revealed that MAL2 is a bona-fide membrane constituent of SVs that is preferentially associated with VGLUT-1-containing nerve terminals. We conclude that SVs specific for different neurotransmitters share the majority of their protein constituents, with only few vesicle proteins showing preferences that, however, are nonexclusive, thus confirming that the vesicular transporters are the only components essential for defining the neurotransmitter phenotype of a SV.

Synaptic and vesicular coexistence of VGLUT and VGAT in selected excitatory and inhibitory synapses.

The segregation between vesicular glutamate and GABA storage and release forms the molecular foundation between excitatory and inhibitory neurons and guarantees the precise function of neuronal networks. Using immunoisolation of synaptic vesicles, we now show that VGLUT2 and VGAT, and also VGLUT1 and VGLUT2, coexist in a sizeable pool of vesicles. VGAT immunoisolates transport glutamate in addition to GABA. Furthermore, VGLUT activity enhances uptake of GABA and monoamines. Postembedding immunogold double labeling revealed that VGLUT1, VGLUT2, and VGAT coexist in mossy fiber terminals of the hippocampal CA3 area. Similarly, cerebellar mossy fiber terminals harbor VGLUT1, VGLUT2, and VGAT, while parallel and climbing fiber terminals exclusively contain VGLUT1 or VGLUT2, respectively. VGLUT2 was also observed in cerebellar GABAergic basket cells terminals. We conclude that the synaptic coexistence of vesicular glutamate and GABA transporters allows for corelease of both glutamate and GABA from selected nerve terminals, which may prevent systemic overexcitability by downregulating synaptic activity. Furthermore, our data suggest that VGLUT enhances transmitter storage in nonglutamatergic neurons. Thus, synaptic and vesicular coexistence of VGLUT and VGAT is more widespread than previously anticipated, putatively influencing fine-tuning and control of synaptic plasticity.